Vol. 4, No. 4 , April 1965
CRYSTAL STRUCTURE OF POTASSIUM SELENOCYANATE 4.99
n
Figure 5.-Packing
diagrams of the 1:l mixture of four SPe2(C0)6 species (left side) and four (S2Fea(C0)9species (right side) in the unit cell.
the symmetry-related trinuclear species. All intermolecular contact distances are greater than 3.1 A., which indicates that this molecular crystal is held together mainly by van der Waals forces. Acknowledgment.-The authors gratefully wish to acknowledge the financial support of the Air Force Office of Air Research and Development Command
(Contract No. AF-AFOSR-5 18-64) and the United States Atomic Energy commission. We are indebted both to the Midwestern Universities Research Association (supported by the Atomic Energy Commission) for the use of the IBM 704 computer and to the Department of Computing Center (University of Wisconsin) for the use of the CDC 1604 computer.
CONTRIBUTION FROM THE DEPARTMENT OR CHEMISTRY, WASHINGTON STATEUNIVERSITY, PULLMAN, WASHINGTON
The Crystal Structure of Potassium Selenocyanate BY DUANE D. SWANK
AND
ROGER D. WILLETT
Received Septembev 10, 1964 The crystal structure of KSeCN has been determined. The compound crystallizes in the monoclinic space group P ~ , / c = 101.13'. h = 7.64 c = 11.89 A., an! with a = 4.59 The structure contains the linear SeCN- ion with a C-N distance of 1.12 A. and a Se-C distance of 1.53 A.
w.:
w.,
Introduction Recently Morgan' has investigated the infrared spectrum of solid KSeCN. His study indicated the existence of the linear SeCN- ion which was slightly perturbed by the influence of the crystal lattice. Al(1) H. W. Morgan, J . Inovg. NucZ. Chem., 16, 367 (1961).
though the SeCN- ion is isoelectronic (in the valence electrons) with the better-known OCN- and SCNions, a crystallographic study of the SeCN- ion had not been made. Because of a theoretical interest in the electronic structure of linear triatomic molecules and ions, it was decided to study the crystal structure of
Inorganic Chemistry
.4ND ROGERD. WILLETT 500 DUANED. SWANK
0
Se
0
K
0C
0
N
of the structure of KSeCN.
Figure 1.-Illustration
TABLE I PARAMETERS FOR KSeCNaib Parameter
Se
Y Pll P22
033 PI2 PlS
023
=
hkl
~ W ( ~ F-, ~I F c 1 ) 2 / ~ ~ ~=F0.086 o12 hkl
Standard deviations are given in parentheses. Anisotropic temperature factors not varied
0.2336 (0.0056) 0.3726 (0.0031) 0.0645(0.0020) 0.0542 (0.0102) 0.0188 (0.0036) 0.0081 (0.0016) 0.0 0.0040 (0.0008) 0.0
c ' ~ F ,-, ~ F , $ / ~ ' F = , ' 0.066 hkl
Rs
0.0028(O.OOO8) 0.0
0.0068(0.0010) =
NC
0,1580 (0,0056) 0.2200 (0.0032) 0,1157(0.0021) 0.0372 (0.0097) 0.0129(0.0034) 0.0056(0.0015) 0.0
0,0003 (0,0007)
XI
hkl
* The p i j are defined by T
KSeCN to verify the linearity of the SeCN ion and to determine the symmetry, if any, required of the ion in the crystalline state. Experimental The compound KSeCh' was prepared in the manner described by Waitkins and Schutt.2 The compound crystallized as needles with the a axis parallel to the n2edle axis. The material is very hygroscopic and decomposes in air to form red selenium and potassium cyanide. Consequently the crystals were placed in Lindemann glass capillaries while the crystallographic data were obtained. The lattice constants for KSeCK, determined from Weissenberg and precession camera photographs using Cu Ka radiation 0.02 A, b = 7.64 0.01 8.,c = (1.5418 A.), are a = 4.59 11.89 =k 0.01 b.,and P = 101.13 2~ 0.08'. Systematic extinc1 for OkO retions ( I = 2% 1 for h01 reflections, and k = 272 flections) imply the space group P21/c. The observed density, measured by a flotation technique, is 2.35 g./cc. The calculated density, for Z = 4, is 2.30 g./cc. Intensity data were collected on a Weissenberg camera with Zr-filtered Mo K a radiation using a combination of multiple film
+
CC
K
0.6198(0.0014) 0.2007 (0,0008) 0.3995 (0,0005) 0.0390 (0.0035) 0,014; (0.0011) 0 . 0 0 i ; (0.0005) -0.0031(0.0015)
0.0273 (0.0006) 0.0434 (0.0004) 0.1973 (0,0002) 0.0456 (0,0015) 0.0115 (0.0004) 0.0068 (0,0002) -0.0022 (0.0008) 0,0075 (0,0004) - 0,0004 (0.0004)
X
+
(2) G. R. Waitkins and R. Schutt, Inorg. Syz. 2, 186 (1946).
=
exp( - P d z 2
- &k2
- &12
- 2&hk - 2Plaltl - 2P2.ikl).
and timed exposure techniques. The zeroth through the third layer were recorded while rotating about the (100) direction. In this manner, the intensities of 491 reflections were recorded. The relative intensities were estimated visually against a set of standard intensities and converted to structure factor magnitudes in the usual manner. Absorption corrections3were also applied to the structure factor magnitudes.
Determination of Structure The Patterson projection onto (100) was calculated first to determine the heavy atom y and z parameters. The largest peaks were assigned as selenium-selenium vectors. Selenium-potassium vector peaks also appeared quite distinctly on the Patterson map. A twodimensional Fourier projection onto (100) based on the postulated Se and K positions, confirmed their assignment. The C and N atoms appeared, though they were not well-resolved a t this point. The x parameters for the Se and K atoms were determined from Patterson projections onto (100) modified by cos (2nu); cos ~
(3) International Tables for Crystallography, Vol. 11, T h e Kynoch Press, Birmingham, England, 1969, pp. 295-298.
Vol. 4, No. 4 , April 1965
+
( 2 ~ 2 ~and ) ) [l cos (2au)I. From these parameters, a trial structure was deduced and refined, the carbon and nitrogen positions being assigned on chemical grounds. A three-dimensional Fourier map confirmed the structure. The Sly, Shoemaker, and Van den Hende ERFR2 IBM 709/7090 program4 was used for the Fourier computations. Atomic positions obtained from these projections were refined with isotropic temperature factors and refinement proceeded rapidly to a value of R1= 0.082 for observed reflections only. Refinement of the anisotropic temperature factors (with the exception of the lighter nitrogen and carbon atoms) further reduced the value of Rl to 0.066 for observed reflections only and to 0.130 for all reflections. The Busing, Martin, and Levy 709/7090 Fortran crystallographic least-squares6 program was used to minimize the function R3as defined in Table I. The intensities for unobserved reflections were assigned one-half the minimum observable intensity and were omitted from the refinement if the calculated structure factor was less than the observed structure factor. A modified version of the Hughes weighting scheme was used as described previously.6 Atomic form factors for K+, C, and N were used as computed from SCF wave functions with exchange effects inc l ~ d e dand , ~ the atomic form factor for Se was used as computed from the Thomas-Fermi-Dirac statistical method.* Final parameters are listed in Table I and a list of the observed and calculated structure factors is given in Table II.9 Bond distances and angles were computed using the Busing, Martin, and Levy 709/ 7090 Fortran function and error prograrn.'O (4) W. G. Sly, D. P. Shoemaker, and J. H. Van den Hende, Esso Research and Engineering Co , Report CBRL-22M-62 (1962). ( 5 ) W. R . Busihg, K. 0. Martin, and H. A. Levy, U. S. Atomic Energy Commission Report ORNL-TM-305 (1962). (6) See R . D. Willett, C. Dwiggens, Jr., R. Kruh, and R. E. Rundle, J . Chem. Phys., 38, 2429 (1963). K Iwas chosen as the average intensity for each layer and Ks as twice the minimum observable intensity. (7) International Tables for Crystallography, Vol. 111, The Kynoch Press, Birmingham, England, 1962, pp. 202-209.
CRYSTAL STRUCTURE OF POTASSIUM SELENOCYANATE
50 1
Discussion This structure contains SeCN- ions which are linear (178.8 i 2.5') to within the accuracy of our determination. The C-N distance is found to be quite short a t 1.117 i 0.026 A., but because of the high standard deviation i t cannot be considered significantly different from a normal C-N triple bond of 1.16 A. The Se-C distance of 1.829 A 0.025 A. is considerably shorter than the normal single bond length of 1.94 A., indicating some multiple bond contribution. This SeCN- ion is isoelectronic in the valence electrons with the OCN- and SCN- ions; however, because of the decreased electronegativity of the selenium atom, i t is expected that the a-bonds will be essentially localized between the carbon and nitrogen atoms. The C-N stretching frequency is smaller for SCN(2048 crn.-I) than for either OCN- (2160 cm.-l) or SeCN- (2070 cm.-I),ll which is not in the order (SeCN- < SCN- < OCN-) expected from electronegativity arguments. However, i t may well be that the force constants, which give a more accurate estimation of bond order, will be in the expected order. Each selenium atom is coordinated to four potassium ions forming a very shallow, distorted rectangular pyramid with the selenium at the apex. The Se-K distances range from 3.4 t o 4.0 8. Each nitrogen atom has three potassium ions as next nearest neighbors with distances ranging from 2.6 to 3.1 A., as illustrated in Figure 1. (8) Reference 7, pp, 210-212. (9) Table I1 has been deposited as Document No. 8197 with the AD1 Auxiliary Publications Project, Washington 25, D. C. A copy may be secured by citing the document number and by remitting $1.25 for photoprints or $1.25 for 35-mm. microfilm. Advance payment is required. Make checks or moriey orders payable to: Chief, Photoduplication Service, Library of Congress (10) W R. Busing, K. 0. Martin, and H. A. Levy, U. S. Atomic Energy Commission Report ORNL-TM-306 (1964). (11) N. N. Greenwood, R. Little, and M. J. Sprague, J. Chem. Soc., 1292 (1964).
